This invention relates to a device for sensing the presence and numerical density of substantially identical articles in a space having predetermined boundaries, and also relates to a conveying system in which the number of articles in the system is regulated by control means which include such a device.
For the purpose of this specification the term "numerical density" is defined as the number of articles per unit volume of the space in question. In the case of a conveying system in which all the articles are directly supported on the same horizontal surface, it will be apparent that numerical density is equivalent to the number of articles per unit area of the surface.
It is a common practice in manufacturing to transport articles such as workpieces from one station to another in a so-called single-file conveying system. In such a system, articles discharged at a given rate from a first operating station are conducted in single file directly to a second operating station which is adapted to receive them at the same rate. In the case of high rates of production, single-file conveying systems must be operated at correspondingly high speeds. They are therefore noisy and may be subject to excessive wear.
Accordingly, mass conveying systems are now employed in many manufacturing operations. In a mass conveying system the articles are continuously agregated at the first station, that is, formed into a mass, and are transported in that mass to the second station or stations. While under normal conditions the rate at which articles are introduced to the conveying system is equal to the rate at which they are withdrawn from it, as in single-file arrangements, the system may be operated at a much lower speed for a given rate of production.
As in all conveying systems, however, a problem arises when input and output rates differ. For example, if the mass conveying system is employed to transport articles from a primary treating or work-performing mechanism to five secondary mechanisms arranged in parallel, it is obvious that the conveying system will remain in a stable condition if each of the secondary mechanisms is operated at one fifth of the rate of the primary mechanism. But if one of the secondary mechanisms should slow for one reason or another, or cease operation altogether, input to the conveying system will exceed output, and the number of articles in the system will rapidly increase.
Most mass conveying systems are provided with excess capacity so that they are able to accommodate momentary abnormal operating conditions. However, if the abnormal condition should persist until total capacity is exceeded, articles may be randomly ejected from the system, or jamming may occur, with consequent harm to the articles, the equipment, or both.
The conventional sensing devices employed to monitor single-file conveying systems are inadequate or wholly unsatisfactory when applied to mass conveying systems. Obviously, devices which make use of such sensors as photoelectric cells or microswitches to count individual articles passing a given point, or to calculate the rate of passage or travel, cannot perform the same functions with any accuracy at all when the articles are aggregated, unless elaborate and costly duplication or refinement is undertaken.
In some mass conveying systems a microswitch or photoelectric cell is located in a portion of the conveying system remote from the normal stream of article movement. When the presence of one or more articles is sensed, indicating that the conveying system has reached capacity or near-capacity, an alarm is energized, or the manufacturing operation is automatically halted.
In any event, efficient, uninterrupted operation of a mass conveying system has required the presence of an operator to monitor the condition of the system continuously and to take corrective action, as necessary, by exercising manual control. Depending on the length or complexity of the system, more than one operator may be required.